The present invention relates to an apparatus or filtering liquids, comprising a hollow body having a wall of filter material, and a stationary support structure on which the hollow body is journalled by means of a first bearing at a first side of the hollow body and a second bearing at a second side of the hollow body opposite said first side, such that the hollow body is rotatable about a horizontal axis, the hollow body forming a circumferential liquid impervious wall, at said second side, extending axially a distance from said wall of filter material and defining a circular opening, which is coaxial with said axis, the support structure forming a liquid supplying chamber partly defined by a stationary circumferential wall, which is close to but spaced from said liquid impervious wall and which at least partly extends along the boundary of said circular opening of the hollow body. There is an inlet to said supplying chamber for a liquid to be filtered. Sealing means are arranged to seal between said circumferential liquid impervious wall and said stationary circumferential wall of the supplying chamber, whereby liquid to be filtered entering said inlet during operation passes the supplying chamber and said circular opening into the interior of the hollow body, from which a filtrate penetrates the wall of filter material. A coarse fraction discharge means is arranged to convey a coarse fraction of the liquid to be filtered from the interior of the hollow body through said supplying chamber out of the apparatus, and a filtrate container is arranged to receive filtrate from the hollow body through said upper opening and has an outlet for discharging filtrate.
This type of prior art filtering apparatus, in which the liquid to be filtered is supplied to the interior of the hollow body so that filtrate flows by gravity through the filter material in the direction out of the hollow body, is suited for filtering a variety of liquids, such as raw water, polluted water and in particular suspensions developed in the pulp and paper processing industry. Coarse particles of undesired material, such as fine fibres and fibre fragments, deposit on the filter material inside the hollow body during operation and are usually removed from top of the hollow body by means of spray nozzles operating from outside the hollow body, so that the coarse fraction discharge means may convey the removed particles out of the apparatus. An important use of this type of filtering apparatus is to clean warm so-called white water obtained in the pulp and paper processing industry. Cleaned warm white-water is re-used as liquid for operating fine cleansing spray nozzles in various parts of pulp and paper processing systems, which reduces the consumption of fresh water and energy.
A problem of this type of filtering apparatus, however, is that the function of the sealing means is difficult to survey. Normally, a minor insignificant liquid leakage past the sealing means occurs during operation, which is acceptable. On the other hand, if the sealing means is more or less damaged or worn the leakage of unfiltered liquid past the sealing means can be quite significant, which of course is not acceptable. As a result, a mal-functioning sealing means can be in operation for a while, so that unfiltered liquid escapes past the sealing means and pollutes the filtrate in the filtrate container. In the case of filtering white-water, fibre residues in the filtrate could clogg said fine cleansing spray nozzles resulting in disturbances in the pulp or paper making process.
The object of the present invention is to provide a filtering apparatus of the above described type, in which the risk of unfiltered liquid polluting the filtrate is eliminated.
This object is fulfilled by the apparatus presented initially, which is characterised in that wall means are arranged to prevent unfiltered liquid leaking past the sealing means from entering the filtrate container.
Said wall means preferably comprises a leakage liquid guiding wall element arranged to guide liquid leaking past the sealing means away from the filtrate container. The leakage liquid wall element may slope in the direction away from the filtrate container.
As an alternative to or in combination with said guiding wall element, said wall means may comprise said circumferential liquid impervious wall formed to extend axially beyond an upper opening of the filtrate container, said sealing means sealing between said stationary circumferential wall and said circumferential liquid impervious wall such that liquid leaking past the sealing means flows by gravity axially outside of said upper opening of the filtrate container.
The support structure may form a leakage chamber arranged to receive liquid leaking past the sealing means. Any liquid received in the leakage chamber may suitably be pumped back to the liquid supplying chamber.
It is preferred that the sealing means comprises a resilient sealing member, which is attached to the stationary circumferential wall of the supplying chamber and which abuts against said circumferential liquid impervious wall of the rotatable hollow body. The sealing member is bent from the circumferential wall of the supplying chamber radially inwardly to said circumferential liquid impervious wall, in order to gain the advantage that hydrostatic pressure exerted from liquid present in the supplying chamber and the hollow body may aid the sealing member to firmly seal against the circumferential liquid impervious wall of the rotatable hollow body.
To make the sealing member capable of adjusting its sealing position so that it seals even if the circumferential liquid impervious wall of the hollow body rotates eccentrically about said horizontal axis, the sealing member suitably abuts against a sealing surface of the circumferential liquid impervious wall which forms an angle to a horizontal line in a longitudinal cross-section through the hollow body which is in the range of 15.degree.-90.degree..
According to one embodiment of the invention, the circumferential liquid impervious wall has a radial inwardly directed flange defining said circular opening, the sealing member abutting against the flange. According to another preferred embodiment of the invention, the circumferential liquid impervious wall has a conical inwardly facing surface, against which the sealing member abuts.
Said second bearing preferably is a slide bearing, the circumferential liquid impervious wall being externally supported by the slide bearing. Suitably, a further sealing means seals between the hollow body and the filtrate container, in order to prevent filtrate from contacting and damage the slide bearing. Said further sealing means suitably seals between a first cylindrical wall element arranged on the hollow body outside of said circumferential liquid impervious wall and a second truncated cylindrical wall element arranged on the filtrate container, or may comprise a resilient sealing member, which is attached to the filtrate container and which abuts against the rotatable hollow body. The support structure may advantageously form a leakage passage arranged to conduct filtrate leaking past said further sealing means to the leakage chamber.
As an alternative, the apparatus of the invention may be provided with the above-described arrangement of liquid supplying chamber, circumferential liquid impervious wall, sealing means, slide bearing and leakage chamber at both sides of the hollow body.